Circuit for regulating current in a PBX loop

ABSTRACT

A fiber optic to electrical conversion unit includes front and rear modular design with rear card modules conditioning signals to be passed to front modules. A frame synchronization method transmits a stream of multiplexing frames and periodically inserts a synchronization word in the stream. Prediction of the next synchronizing word in the receiving unit reduces the need to search for a synchronization word in the stream. Loop current regulation in a two-wire loop between a central office and a PBX uses a transformer serially connected in the two-wire circuit with a regulation circuit which holds the current in the loop within a predetermined range to permit the transformer to be held to a relatively small size. High frequency noise bypassing is accomplished by forming capacitor loops from the power connection to the ground connection of the component near to the component.

This is a continuation of application Ser. No. 07/204,751, filed June10, 1988, now abandoned.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field oftelecommunication and more particularly to the art of electrical tooptical signal conversion.

BACKGROUND OF THE INVENTION

In U.S. Pat. No. 4,751,613 to Werdin and Henke, issued June 14, 1988,there is disclosed a low RF emission fiber optic transmission system. Inthat system there is provided a conversion unit which convertselectrical signals to fiber optic signals and vice versa. The system canthus provide a secure fiber optic link between electrical systems. Inaddition, the conversion unit is designed to suppress and contain RFemissions from the conversion circuitry.

The system described in U.S. Pat. No. 4,751,613 is modular and permitsspecialized conversion modules to be mixed and matched. Modules formultiplexing and de-multiplexing RS232-type data channels, test modules,power supply modules, high speed data conversion modules, voicefrequency multiplexing modules and telephone interfacing modules are allcontemplated by the system.

The present application discloses a modification the conversion unitdescribed in U.S. Pat. No. 4,751,613, together with certain additionalnew circuits and circuitry features for use in the conversionelectronics thereof. More particularly, there is disclosed a conversionunit having front and rear modularity, a design methodology forimplementing bypass capacitors to minimize RF emissions from the powerlead of circuit components, a loop current regulating circuit for a PBXmodule and a voice frequency multiplexing circuit and

SUMMARY OF THE INVENTION

According to the invention, there is provided a circuit for regulatingloop current in a two wire loop between a central office and a PBXemulator. The circuit includes a transformer means for serial connectionin said two wire circuit, and means for actively regulating the currentin said loop so that it remains within a predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the conversion unit according to thepresent invention;

FIG. 2 is a side view of the conversion unit according to the presentinvention;

FIG. 3 is a top view of the conversion unit according to the presentinvention;

FIG. 4 is a front view of the conversion unit according to the presentinvention;

FIG. 5 is a rear, view of the conversion unit according to the presentinvention;

FIGS. 6, 7 and 8 are side, top and rear views of a rear conversion cardmodule according to the present invention;

FIGS. 9a through 9e are schematic diagrams of applications of thefront-rear modularity according to the present invention;

FIGS. 10 and 11 are schematic block diagrams of the transmitter andreceiver sections of the voice frequency multiplexing circuitryaccording to the present invention;

FIGS. 12a through 12i are detailed schematic diagrams of the voicefrequency multiplexing circuitry according to the present invention;

FIG. 13 is a block diagram of the PBX module according to the presentinvention;

FIG. 14a-14b is a detailed electrical schematic of the circuit forregulating loop current in a loop between a central office and a PBXemulator;

FIG. 15 is an electrical schematic diagram of a typical component of amodule according to the present invention; and

FIG. 16 is a perspective drawing showing the positioning of signaltraces and bypass capacitors on a layout according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION Front-Rear Modularity

Referring now to FIGS. 1 through 8, there is shown a conversion unit 10with front and rear modularity according to the present invention.Conversion unit 10 is a modified version of the conversion unitdescribed in U.S. Pat. No. 4,751,613, the entire disclosure of which ishereby incorporated herein by reference. The interface unit 10 includesa housing/chassis 12 having top and bottom walls 14 and sidewalls 16. Amotherboard 60 is mounted to extend across the rear end of the chassisin order to generally define a forward signal containment area 22 and arear signal containment area 22'. A panel 20 with holes for fiber cablesextends across the top of motherboard 60. The front end of area 22 andrear end of area 22' are covered by the various front panels 100 and100' of card modules 50 and 50', respectively, or with blanks, as may beseen with respect to the views of FIGS. 4 and 5.

As may be seen best with respect to FIGS. 1 and 2, the chassis 12includes a plurality of card guides 30 which are formed from a singlesheet of sheet metal and fixed to the top and bottom walls 14 to providecooperating guides. As may be seen with respect to FIG. 1, a card module50 may thus be mounted in the chassis 12 by sliding it in the front ofthe chassis. Each card module 50 comprises a printed circuit boardmounted to a sheet metal support of roughly the same outline so that thesheet metal support slides in the card guides. The front panels 100 ofeach of card modules 50 are welded to the metal support, and the printedcircuit boards are mounted to the support with metal stand-offs.

As may be generally seen with respect to FIGS. 1, 2, and 3, motherboard60 includes a plurality of connectors 62 for receiving a mating DINconnector 64 of the modules 50. Motherboard 60 provides for power bussupply to the modules 50 as supplied from a power supply module 51, forexample, and any other necessary bussing or connection between modules.Thus, a module 50 inserted in the chassis 12 mates with a correspondingslot in the motherboard 60.

As may be seen with respect to FIGS. 2 and 3, certain of connectors 62are wired through motherboard 60 to corresponding connectors 62' on theother side thereof. Connectors 62' face toward a rear area 22' of thechassis 12, and are each provided to receive the connector 64' of a rearcard module 50'. The remaining connectors 62 may be female DINconnectors, to be used, for instance, where no rear modularity isrequired. Alternatively, as provided for in the chassis unit describedin U.S. Pat. No. 4,751,613, connectors 62 are preferably 96 pin DINconnectors with a male having long tails which can be used to connect tooutside data lines.

Preferably, the rear end of chassis 12 is designed in the same manner asthe front end of chassis 12 to provide RF containment and suppressionwithin the rear of the chassis; provided, however, card guides are notrequired because of the short length of the rear area 22'. Card modules50', like modules 50, each include a printed circuit board mounted witha standoff to a sheet metal support, and a front panel 100' welded tothe metal support. Modules 50', however, need not necessarily utilizethe multilayered design of modules 50, as explained further below. Inaddition, each of modules 50' include a shielded connector 51' and anRFI waveguide 52'. Incoming and outgoing electrical signal lines andbuses can be connected to the conversion circuitry through connectors51', and fiber optic cables can be passed through waveguides 52'. Thus,the present invention modifies the conversion unit disclosed in U.S.Pat. No. 4,751,613 so that the front and rear of the unit are generallyfunctionally symmetrical, except, however, that the rear signalcontainment area 22' and modules 50' are smaller than forward area 22and modules 50, respectively. Further details on the preferred design ofthe front end of unit 12 may be found in U.S. Pat. No. 4,751,613. Inparticular, there are disclosed in U.S. Pat. No. 4,751,613 furtherdetails concerning the multi-layer circuit board technique, groundingtechniques and circuit board mounting techniques and housingconstruction methods which play a part in the RF suppression strategy.

Referring now to FIGS. 9a through 9e, there is shown in schematic formvarious implementations of the front-rear modularity of conversion unit10. In each of FIGS. 9a through 9e, circuit module 50 is represented ascontaining conversion circuitry for converting electrical signals tofiber optic signals and vice versa. For instance, the conversioncircuitry may consist of multiplexing circuits for multiplexing aplurality of electrical signals onto a single fiber, digitizing circuitsfor digitizing analog electrical signals for transmission in digitalfiber optic protocol, and various other circuits for convertingelectrical control signals to fiber optic signal counterparts, as neededto signal through the fiber optic link. As also shown in FIGS. 9athrough 9e, rear module 50' may be configured to perform complementarysignal processing functions such as filtering, waveshaping, protection,level conversion, or may simply consist of a straight through connection(FIG. 9a). Preferably, the signal processing functions carried out onrear module 50' do not involve high speed digitizing and multiplexingfunctions which normally give rise to the greatest source of RF emissionin the conversion unit. Thus, module 50' may be formed of only twolayers where source suppression of noise is not required.

The front-rear modular design of conversion unit 10 provides that theconversion circuitry of modules 50 may be readily adapted to interfacewith a variety of different electrical interfaces by providing customrear modules 50'. Thus, front modules 50 may be standardized to agreater extent than otherwise possible, and mixed and matched with rearmodules 50' to be adapted to various electrical interface systems. Inaddition, the front-rear modularity system provides that card modules 50may be more readily adapted to custom applications because rear modules50' preferably perform only a few functions and thus need only twolayers, in most cases, may be more readily redesigned and produced.

Rear modules 50' may employ the multi-layered circuit board design setforth in U.S. Pat. No. 4,751,613 where desirable in noise generatingcircuit designs. It shall be understood, however, that the front-rearmodularity concept is in no way limited to application in conversionunits which require RF containment and suppression. Rather, thisfront-rear modularity may be readily employed in other electricaldevices using circuit card modules in which it may be desirable toprovide auxiliary signal processing.

VOICE FREQUENCY MULTIPLEXER

Referring now to FIGS. 10 and 11 there is shown in schematic blockdiagram form the transmitting and receiving paths, respectively, of thevoice frequency multiplexing circuit module according to the presentinvention.

The voice frequency multiplexing circuit contains circuitry to convertanalog information from four independent four-wire VF interfaces withE/M signalling into a digital format. The voice frequency multiplexermultiplexes this digital information into one single serial channel.This signal is converted into light waves which are transmitted overfiber optic cable. Received light waves from a remote multiplexingcircuit are converted back to the four-wire VF interfaces with E/Msignalling on the same circuit module. In addition, the voice frequencymultiplexer circuitry provides electrical monitoring via 16 connectors.The circuitry further preferably includes local loopback of the remotelygenerated fiber optic signal and VF aggregate loopback of the serialdata stream.

Various abbreviations are used with respect to the description of FIGS.10 and 11, as follows:

    ______________________________________                                        Abbreviations                                                                 ______________________________________                                        BCLKX         Bit clock transmit                                              BCLKR         Bit clock receive                                               DX            Data transmit PCM                                               DR            Data receive PCM                                                FSX           Frame synchronization transmit                                  FSR           Frame synchronization receive                                   E/M           Ear/mouth signaling                                             FP            Frame pulse                                                     FSS           Frame synchronization signal                                    SCHR          Synchronization channel receive                                 CTS           Clear to send                                                   NVM           Non-valid manchester code                                       CO            Clock out                                                       BOO           Bipolar one out (Manchester)                                    BZO           Bipolar zero out (Manchester)                                   VCHR          Voice Channel Receive                                           VCHT          Voice Channel Transmit                                          VF            Voice frequency                                                 PDN           Power down                                                      SWD           Synchronization word detect                                     ______________________________________                                    

VF Interface (transmit/receive)

A codec/filter 210 provides analog to digital and vice versa voiceconversion. The codec 210 combines coder/decoder with internal switchedcapacitor filters for signal conditioning. Operation of the codec 210receive Section 212 and transmit section 214 is fully asynchronous.Transmitted bit rate for the NRZ data is 384 bps. Transmit frame pulserate is 76.8 kHz.

Manchester Transmitter/Receiver

A Manchester Biphase Encoder 220 converts the multiplexed serial NRZdata into an appropriate biphase signal. The manchester coder/decoder220 consists of a transmit section 218 and a receive section 216. Thedecoder converts the serial biphase signal into a serial NRZ datastream. All timing references BCLKR and BCLKX within the multiplexercircuit 200 are derived from the serial biphase coder/encoder clockoscillator. Biphase masterclock is 12.288 MHz. NRZR and NRZ dataencoding clock is 384 kHz. The decoder also provides the bit clock forthe received NRZ data.

Signal Word and Synchronization Word Generation

Insertion of a repetitious synchronization word and status dependentsignaling word is achieved with an eight bit parallel load shiftregister 225. The output of this shift register is TRI-state bufferedand is connected to the transmit (DX) rail. Because this registerreceives the same framing information and operates in the transmit modeas the voice codecs, it can be viewed as a transmit section of a digitalcodec, thus providing a fifth channel.

The transmit frame address generator 230 provides time slot allocationfor the framing of the five channels. The generator operates in a freerunning mode and has the following time sequence:

"CH5", CH1", CH2", CH3", CH4"

Channel 5 is the Synchronization and Signaling channel (hereinafterreferred to as the S+S channel). Each frame consists of 8 bits. The S+Schannel is divided into two sections, four bits for synchronization andfour bits for signaling information for the four corresponding channels.

Manchester Synchronization Initialization Sequence Control

The Manchester synchronization initialization sequence circuit 235provides the manchester transmitter/receiver with an appropriate timingsequence for its clear to send, hereinafter referred to as CTS, controlinput. A falling edge at the CTS input will cause the manchestertransmitter to send a short synchronization initialization sequencewhich in return should be recognized by the remote manchester receiver,if connected. The CTS input will be toggled by the sequence control, andtherefore cause the manchester transmitter to issue a synchronizationsequence until a valid local manchester synchronization initializationsequence has been detected and is present at the NVM output. Asynchronization sequence is also issued upon power-up reset.

Reset, Control and Fault Indications

The reset, control and alarm logic 250 generates power-up reset signals,manchester initialization sequence signals, loss of synchronizationindication.

Receive Synchronization/Signal-Word Monitor

The incoming decoded NRZ bit stream is being clocked into an 8 bitserial-in/parallel-out register 260 with the decoded bit clock. Theoutputs e.g. A, B, C, D are dedicated to the four signaling bits, e.g.E, F, G, H are dedicated to the synchronization word detection. Receiversynchronization is achieved by monitoring the decoded NRZ bit stream andlooking for the matching 4 bit (E.F.G.H) synchronization word. Upondetection of a bit pattern which matches the code of the synchronizationword a detection pulse is generated.

Receive Frame Synchronizer and Frame Counter

A receive frame synchronizer 270 and frame counter are also provided.When detection of the first synchronization detect pulse occurs theforecast counter 275 is initialized to predict the position for thesynchronization word within the next frame. The forecast counter 295operates as a frame window generator to allow the detection of asynchronization word only at a certain time and frame instant. In orderto provide reliable synchronization with a 4 bit word the synchronizer270 must average over a period of incoming synchronization frames. Thiscan be accomplished with the forecast counter 295 in conjunction with anup/down counter. The incoming NRZ bit stream is being clocked with thedecoded bit clock into the receive sync/signal word monitor 260. A 4 bitmagnitude comparator monitors the output of the register for equivalencewith the synchronization word code. A synchronization word detect pulsefor the duration of a clocking pulse indicates the reception of asynchronization word. If the up/down counter (part of 270) is at itslowest possible count, indicating a loss of synchronization, theforecast counter 275 will then be reset by the synchronization detectpulse and the up/down counter advances one step up. Now the forecastcounter 275 is able to provide the frame window for the prediction ofthe postceding synchronization word. This method reduces the possibilityof falsely causing the synchronizer 270 to lock on a pseudosynchronization word, generated by any of the residual frames. Detectionof a preceding synchronization word will advance the up/down counteragain. Detection of no synchronization word at the predicted time slotwill retard the up/down counter.

Count advancing from the lowest (loss of synchronization point) to fullsynchronization is accomplished in sixteen levels:

    ______________________________________                                        level 1:  0 count, full loss of synchronization                               level 2:  1st count, 1st predicted sync word recognized                       level 3:  2nd count, 2nd predicted sync word recognized                       level 4:  3rd count, 3rd predicted sync word recognized                       level 5:  4th count, 4th predicted sync word recognized                       level 6:  5th count, 5th predicted sync word recognized                       level 7:  6th count, 6th predicted sync word recognized                       level 8:  7th count, lowest level of synchronization                          level 9:  8th count, lowest level of synchronization                          level 10: 9th count, lowest level of synchronization                          level 11: 10th count, lowest level of synchronization                         level 12: 11th count, mid level of synchronization                            level 13: 12th count, mid level of synchronization                            level 14: 13th count, mid level of synchronization                            level 15: 14th count, mid level of synchronization                            level 16: 15th count, top level of synchronization                            ______________________________________                                    

In case that the synchronization word cannot be detected at theforecasted position, the up/down counter will be retarded for one count.This will happen until the zero count is reached. Only at the full lossof synchronization (counter zero count) the forecast counter can bereset upon reception of a synchronization word for a new timing windowposition.

The forecasted word synchronization method described has variousadvantages, as follows:

(a) reliable synchronization once synchronization has been detected

(b) high synchronization signal to noise ratio due to the relative slowaveraging synchronization mechanism

(c) bit errors within signal or few synchronization words do not disruptmultiplex synchronization

(d) allows transmission of unrestricted digital information

(e) fast resynchronization times (typically less than 1 ms)

During power down or initialization sequence the codecs transmit in idleor standby bit pattern which does not contain a steady stream ofsynchronization words. This feature ensures a secure resynchronizationafter power down signal periods. Any loss of synchronization will mutethe codecs and force the idle pattern to be transmitted. Also, allsignaling bits are forced to a non-active state.

Signaling Decoder Converter

A signaling decoder converter 300 is provided. Signal bits (A,B,C,F) arelatched and fed to the output signal ports at mid and top level of thesynchronizer position. The TTL level is converted to the E and M levels.

Receive Frame Address Generator

A receive frame address generator 310 is also provided. This addressgenerator provides the time slot allocation between the five receivechannels of the multiplexer. The generator is controlled by the outputof the forecast counter. Each forecast count pulse will initiate theknown transmit sequence:

"CH5", CH1", CH2", CH3", CH4"

Note: Channel 5 is the synchronization and signaling channel andtransfer the appropriate timing signal to the corresponding codecreceive section.

Further details of the design of the voice frequency multiplexercircuitry may be found in FIGS. 12a-i, which are detailed schematics ofa preferred form of said circuitry. Where the voice frequencymultiplexer circuit is utilized in an environment requiring low noise,it shall be understood that it would be preferred to employ it using themulti-layered circuit board design disclosed in U.S. Pat. No. 4,751,613.It shall be understood, however, that the application of the circuit isin no way limited to such application.

CIRCUITRY FOR REGULATION OF LOOP CURRENT BETWEEN CO AND PBX EMULATOR

Referring now to FIG. 13 there is shown a schematic block diagram of thePBX interface circuitry 400 according to the present invention. The PBXmodule 400 contains circuitry to convert four two-wire loopstart VFcircuits into four four-wire VF with E/M type signaling. Module 400 isused in connection with the voice frequency multiplexer, which takes thefour four-wire VF output channels and multiplexes them over a singlefiber optic cable. As illustrated in FIG. 13, module 400 thus performsthe following functions:

(a) two-wire to four-wire hybrid function

(b) secondary overvoltage protection

(c) switch hook detection

(d) loop current regulation

(e) ring detection

(f) E/M signaling translation

(g) signal isolation,

(h) FCC 68 compatibility.

Referring now to FIG. 14 there is shown a detailed schematic diagram ofone of the channels of PBX module 400. Channel circuitry 410 receivesTip (T) and Ring (R) signals on conductors 412 and 414, respectively.The circuitry within the phantom line box 420 provides means fordetecting ringing on the circuit and converting it to a ring controlsignal 422. A relay 430 is provided, and is controlled by an incomingswitch hook detect signal SHD. Transformers T1 and T3 are providedwhereby signals carried on conductors 412 and 414 are picked up byamplifier 435 and output on conductor 436 and whereby incoming signalson conductor 438 are conveyed through amplifier 440 and transformer T3onto Tip and Ring conductors 412 and 414.

The aspect of circuit 410 of particular interest to the presentinvention is the loop current regulation circuit contained within thephantom line box 450. This loop current regulation circuit comprisestransistor 451, resistor 452, low power voltage reference 453, capacitor454 and resistor 455.

Regulation of the loop current in this particular application providesthe PBX interface module (or PBX emulator) with a much narrower andcontrolled current range than would normally be found in suchapplications (20 mA to 120 mA typical). Application of the loop currentregulator limits this range to approximately 60% (23 mA to 40 mAtypical). Lowering the loop current range allows the use of muchsmaller, more reliable and cheaper audio transformers 456 and 457.

The loop current regulator is based on that of a constant direct current(DC) source type regulation. Capacitor 454 provides alternating current(AC) bypassing for voice type signals.

Current regulation is accomplished by maintaining a constant voltageover node 2 and node 3 using a wide range reference diode 453. Thereference diode 453 regulates the voltage over node 2 and node 3 toapproximately 1.2 v over a current range of 500 uA to 50 mA. Forwardcurrent to the reference diode 453 is limited by resistor 452.Transistor 451 operates in the linear mode. The base of transistor 452(node 2) is regulated by the reference diode to 1.2 v. The forward biasvoltage drop over the base to emitter diode of transistor 451 isapproximately 0.7 v. This base to emitter forward bias voltage willchange with temperature but this voltage drift can be tolerated due tothe non-critical regulation requirement. The constant voltage drop overresistor 455 can be calculated to 0.5 v (1.2 v-0.7 v). Using a 22 Ohmvalue for resistor 455, the resulting constant current can be calculatedto approximately 23 mA (0.5 v/22 Ohms).

Thus, the application of a wide range constant current source in thiscase reduces transformer size and thus increases packing density.

In implementation, the PBX module 400 is connected with a voicefrequency multiplexing circuit as described above. The voice frequencymultiplexing circuit receives the switch hook detect, ring controlsignal and voice frequency signals from the circuitry 410 of the PBXmodule whereby a two-wire PBX interface is provided for each channel ofthe four channel voice frequency multiplexer module. Where the PBX model400 is utilized in an environment requiring low noise, it shall beunderstood that it would be preferred to employ it using themulti-layered circuit board design disclosed in U.S. Pat. No. 4,751,613.It shall be understood, however, that the application of the currentregulating circuitry is in no way limited to such application.

METHOD FOR BYPASSING HIGH FREQUENCY NOISE FROM THE POWER CONNECTION OFCOMPONENTS

Referring now to FIG. 15 there is shown an electrical schematic of anexample of an input stage of a conversion module which may be used inthe conversion unit of the present invention. Circuit 500 may comprise,for instance, an input buffer for a high speed conversion module.Circuit 500 includes a buffer amplifier 510 connected to a source ofpower VCC through a 22 UH inductor element and to a signal ground 511.

As is conventional in the prior art, several capacitors C2, C3 and C4 ofvarying capacitances are provided to bypass the various high frequencynoise components emitted from the power connection 515 of amplifier 510as it operates. Capacitor C1 is shown in phantom and represents thecapacitance inherent in the PC board layout. In operation, eachcapacitance is selected to bypass a certain frequency spectrum of thenoise generated by connection 515, whereby RF emission is minimized. Forexample, in a high speed conversion module or circuit 500 isimplemented, the input signal to buffer amplifier 510 would be in thespectrum of frequency usage range.

Referring now to FIG. 16, there is shown a schematic diagram of thepreferred physical layout of capacitors C2, C3 and C4 in a PC boardaccording to the present invention. In FIG. 16, VCC is shown carried ona power plane 503, with the signal ground on a ground plane 505. Traces507 and 508 run on a signal routing layer, and connect component 510 tothe power plane 503 (optionally through an indicator 509) and groundplane 505, respectively. Where the multilayer noise suppression PC boardconstruction technique disclosed in U.S. Pat. No. 4,751,613 is used,power plane 503 and ground plane 505 would sandwich the PC layercarrying traces 507 and 508. Nodes 520a and 520b, 521a and 521b and 522aand 522b represent the points where capacitors C2, C3 and C4 arephysically connected to traces 507 and 508. The connection of trace 507to the VCC power trace is accomplished at the end 525 of trace 507,while the connection of trace 508 to ground plane 505 is accomplished atthe end 526 of trace 508. These connections go through the PC board tothe respective layers 503 and 505.

For optimum bypass operation, it has been found that a physical locationof nodes 520a and 520b, 521a and 521b, and 522a and 522b betweencomponent 510 and the VCC and ground plane connection traces 507 and 508of a substantial impact on the effectiveness of the bypass operation.Accordingly, the present invention calls for physically locating nodes520a, 521a and 522a between end 525 and component 510 along the path oftrace 507, and nodes 520b, 521b and 522b between end 526 and component510 along trace 508. Thus, noise which is bypassed by capacitors C2, C3and C4 tends to loop back to the ground connection 511 of component 510and not be transmitted to the VCC or ground planes 503 and 505,respectively. Moreover, it is preferable that the capacitors C2, C3 andC4 be arranged so that the one bypassing the highest frequencycomponents be located in the innermost position. In FIG. 15, this iscapacitor C2.

Thus, there has been described above various method and apparatusrelating to the conversion of electrical to fiber optic signals. Inparticular, there has been described a front-rear modular conversionunit, a voice frequency mutliplexing circuit, a circuit for regulatingloop current between a CO and a PBX emulator, and a method forminimizing high frequency noise radiation from a circuit board usingbypass capacitors. Although these inventions have been described intheir preferred form, those skilled in the art will readily recognizethat many changes and modifications may be made thereto withoutdeparting from the spirit and the scope of the invention, as set forthin the claims appended hereto.

What is claimed is:
 1. Apparatus for interfacing subscriber equipment totip and ring lines originating from a central office source,comprising:a tip connection for connection to said tip line; a ringconnection for connection to said ring line; transformer means connectedserially between said tip and said ring connections to form a subscriberloop, said transformer means for applying and extracting AC signals toand from said loop, the DC current in said loop sourced from the centraloffice; regulator means for actively regulating the DC current of saidloop so that the loop current is maintained substantially within apredetermined range which is narrower than the unregulated range of loopcurrent that could exist in the absence of said regulator means; andsaid transformer means having an operating range corresponding to saidpredetermined range of loop current and narrower than the operatingrange required for the unregulated range.
 2. Apparatus according toclaim 1 further wherein said regulating means is voltage responsive andis connected in series with said transformer means.
 3. Apparatus forinterfacing subscriber equipment to tip and ring lines originating froma central office source, comprising:a tip connection for connection saidtip line; a ring connection for connection to said ring line; at leastone transformer winding connected between said tip connection and saidring connection to form a subscriber loop wherein the current in saidloop is sourced from the central office; a transistor in series withsaid transformer winding; circuit means for biasing said transistor intolinear operation; and voltage reference means coupled to said transistorfor establishing an operating point; whereby said transistor modulatessaid loop current about said operating point.
 4. The apparatus of claim3 wherein said transistor is an NPN device with the collector coupled tosaid tip connection, and the emitter coupled to said ring connection. 5.The apparatus of claim 3 wherein said voltage reference means includes azener diode.
 6. The apparatus of claim 5 wherein said circuit means forbiasing includes a load resistor in series with said emitter.
 7. Theapparatus of claim 6 wherein said circuit means for biasing includes acurrent limiting resistor in series with said zener diode for supplyingbase current to said transistor.
 8. A method for interfacing subscriberequipment to tip and ring lines originating from a central officesource, comprising the steps of:(a) connecting transformer means betweensaid tip and ring lines to form a subscriber loop, the DC current insaid loop sourced form the central office; (b) applying and extractingAC signals to and from said loop using said transformer means; (c)actively regulating the DC current of said loop from the subscriber endof said loop so that the loop current is maintained substantially withina predetermined range which is narrower than the unregulated range ofloop current that could exist in the absence of said regulation; and (d)selecting said transformer means to have an operating rangecorresponding to said predetermined range of loop current such that saidoperating range is narrower than the operating range required for theunregulated range.
 9. A method according to claim 8 wherein the currentis regulated according to the voltage across the tip and ring lines.